(1) Field of the Invention
The present invention relates to a metal oxide semiconductor (MOS) type output driver circuit, more particularly, to a protective circuit for an output transistor of the output driver circuit during short-circuits.
(2) Description of the Prior Art
N-channel MOS type output driver circuits have recently come into wide use for output driver circuits in address buses and data buses of microcomputers and the like. N-channel MOS type output driver circuits are usually used in bidirectional buses. Therefore, if one output driver circuit erroneously outputs an "H" level when another output driver connected to the same bus line outputs on "L" level, the output transistors of the output driver circuits would be destroyed by overcurrent. This makes it necessary for the output driver circuits to be replaced every time such troubles occur.
Further, quasi-birdirectional bus sytems recently have come into general use as address buses and data buses of microcomputer systems. In quasi-bidirectional bus systems, the output driver circuits connected to the bus line are not controlled in the high impedance state, unlike ordinary bidirectional bus systems. When one of the output driver circuits connected to the same bus line outputs an "L", level the outputs of the other output drivers, which is set an "H", level must be pulled down to the "L" level without damage.